A sewage spill that flowed into the Minnesota Valley National Wildlife Refuge from a blocked sewer line in Burnsville will undergo final cleanup Thursday.

The rung of a ladder apparently left in a manhole broke off and wedged in the sewer line last month, the city said. The ladder part collected debris and blocked the line, forcing sewage to well up in the manhole. The sewage leaked out into a wetland that adjoins Black Dog Lake, along the Minnesota River.

Refuge officials are concerned about a possible fish kill and more algae in the lake, but it’s too soon to tell what the damage will be, said Jeanne Holler, deputy refuge manager. “It’s difficult to quantify it now, but definitely raw sewage can be a toxic to aquatic life. It’s not good to have that in the wetland and in the whole aquatic system.”

How much sewage escaped is unclear because it’s not known how long the manhole was leaking; it was at least a couple of days.

About 6 to 8 cubic yards of solids will have been removed from the wetland grounds with shovels and skid-steer loaders, said Linda Mullen, Burnsville utilities superintendent. An estimate of how much liquid escaped is harder to determine. Using city measures of how much sewage the pipe carries per minute, the wildlife refuge estimated the flow to be about 44,000 gallons a day of both liquids and solids.

Liquid from the spill flowed across the wetland into a ditch that empties into a trout stream, then to the lake.

A railroad worker noticed a bad smell near Hwy. 13 and Cedar Avenue during a track inspection on Feb 21. Workers from the city of Eagan spent a day looking for the leak, and then Burnsville began to search, finally stopping the leak on Feb. 23.

Though Burnsville was concerned about locating the leak and stopping it, Holler said both the city and the wildlife refuge should have acted more quickly to contain the liquid and prevent it from flowing into the lake.

But because the lake’s primary purpose is to provide cooling water for Xcel Energy’s Black Dog power plant, the water will be “circulating more than any other lake might be at this time of year,” and that will help prevent the sewage from settling in one spot, Holler said.

The sewer line in question serves 130 homes and runs eight feet under the wetland from Burnsville to the Seneca Wastewater Treatment Plant in Eagan. There was no damage to the pipe.

The cleanup cost Burnsville about $17,000.